Bisimides, such as the bismaleimides and their 3-methyl analogues; the biscitraconimides, are being increasingly used as matrix resins for fiber-reinforced composites, especially high-performance graphite fiber-reinforced composites. The resins are cured by a thermally induced addition reaction to give highly crosslinked, void-free network polymers having good physical properties with higher thermal stability, higher char yield, better fire resistance and lower water absorption than currently-used epoxy systems.
High performance graphite-fiber reinforced, organic matrix composites are considerably lighter than metals and are used to replace metals in many aerospace applications. Bismaleimides are one of several polymer candidates being examined for use as resin matrixes for fiber-reinforced composites usable at elevated temperatures. Low molecular weight bisimide prepolymers, end-capped with reactive maleimide rings are polymerizable thermally into highly cross-linked heat-resistant polymer networks without formation of troublesome or hazardous byproducts. A general discussion of this field is found in the following references: W.J. Gilwee et al., SAMPE Symposium, Vol. 16, p. 284 (1973); F. Grundschober et al., U.S. Pat. No. 3,380,964; A. Bergain et al., U.S Pat. No. 3,562,223; G.T. Kwiatkowski et al., J. Polym. Sci. Chem Ed. Vol. 13, p. 961 (1975).
However, there remain some problems with the bisimide-type resins: for example, difficult processing conditions, solvent retention in the prepregs, high melting point and high curing temperatures of the monomer, and brittleness of the final polymers because of the high cross-link density obtained in network polymers.
Recently, several modifications have been disclosed of bisimide-type resins that improve the resins' properties or their processibility. These include a hot-melt bismaleimide described by H.D. Stenzenberger, et al., SAMPE Symposium, Vol. 29, p. 1043 (1984) and the modification of bismaleimide with vinyl or allyl comounds to improve the cure temperature and toughness of the resin [See M.S. Hsu et al. SAMPE Symposium, Vol. 29, p. 1034 (1984) and J.J. King et al, SAMPE Symposium, Vol. 29, p. 392 (1984)].
In U.S. Pat. No. 4,269,966, Stenzenberger discloses novel polyimide prepolymers which are produced by reacting an unsaturated dicarboxylic acid imido acyl chloride with a difunctional amine to produce the corresponding acid amide. The condensation of these monomers is preferably performed in solution in a low boiling solvent. The resulting prepolymer is hardened and completely polymerized by heating, preferably between about 80 and 400.degree. C. to produce a cross-linked, substantially infusible and insoluble polyimide resin. Stenzenberger generally discloses a number of the bisimides used in this invention, but does not disclose that mixtures of the specific bisimides give products which have enhanced physical and thermal properties as is found in the present invention.
Bonnard, in U.S. Pat. No. 3,627,780, discloses the preparation and use of bismaleimides. However, these bismaleimides do not have additional amide structures between the maleic acid moieties as is found in the present invention. In U.S. Pat. No. 2,745,841, Tawney et al., also discloses a number of bismaleimides which do not incorporate additional amide linkages between the maleic acid moieties.
Additional U.S. patents of general interest which disclose polyimide compositions having bismaleimide terminal groups include: U.S. Pat. Nos. 4,418,181 (to Monacelli), 3,883,609 (to Ladd), 3,887,582 (to Holub et al.), and 4,280,948 (to Darms et al.)
This body of art reflects the need for bisimide resin systems which exhibit improved processibility and improved physical and mechanical properties as are described in the present invention.